Normal adult haemoglobin consists of a tetramer of four haemoglobin chains, two α-chains and two β-chains. O2 binds to the tetrameric form of haemoglobin and is transported in the blood. Fetal blood comprises fetal haemoglobin, a tetramer consisting of two α-chains and two γ-chains. Further haemoglobin chains have been identified, such as δ-chains, ε-chains, zeta-chains, τ-chains or the S form known to be the mutation seen in haemoglobin of individuals suffering from sickle cell disease.
Intravascular lysis of red blood cells (haemolysis) leads to the release of haemoglobin into plasma. This phenomenon occurs during physiological as well as pathological conditions. Pathological complications are severe when accelerated in infectious e.g. malaria), inherited (e.g. side cell anemia), or autoimmune diseases. The haemoglobin tetramers are converted to haemoglobin dimers capable of binding haptoglobin. In the plasma haemoglobin is captured by the acute phase protein haptoglobin. Haptoglobin is a blood plasma protein having a molecular weight of approximately 86,000 to 400,000 and plays an important role in the metabolism of haemoglobin liberated into the blood stream. When liberated excessively in the blood the haemoglobin is excreted into the urine through the renal tubules, resulting in not only an iron loss but also disorders of the renal tubules. Because haptoglobin binds selectively and firmly to haemoglobin in vivo and thereby forms a haemoglobin-haptoglobin complex, it has important functions in the recovery of iron and in the prevention of renal disorders.
Hp is synthesised as a single chain, which is post-translationally cleaved into an amino-terminal α chain and a carboxy-terminal β chain. The basic structure of Hp, as found in most mammals, is a homodimer (FIG. 2a), in which the two Hp molecules are linked by a single disulfide bond via their respective ˜9 kDa α chains. In man, a variant with a long α chain is also present in all populations. This variant arose apparently by an early intragenic duplication, presumably originating from an unequal crossover of two basic alleles, resulting in an Hp with an α chain of ˜14 kDa. The short and long α chains are designated as α1 and α2 respectively. Since the cysteine forming the intermolecular disulfide bond between the α chains is also duplicated, humans carrying the long variant allele exhibit a multimeric Hp phenotype (FIG. 2a).
Conventional human haptoglobins have been well studied; they were discovered over 40 years ago and their role is thought to be in the plasma transport of free haemoglobin. Additionally, haptoglobin is believed to have anti-inflammatory activities, such as its decreasing effect on neutrophil metabolism, and an effect on the immune system by possibly modulating B cell proliferation and decrease antibody production. The mechanisms of the influence of haptoglobin on immune function is unknown. The potential signalling pathways by which haptoglobin is mediating its effects, and the existence of a haptoglobin receptor have not been disclosed in the prior art.
However, Ghmati et al., 1996 describe a study in which haptoglobin is an alternative low-affinity ligand for CD11b/CD18 on monocyte cell lines. CD11b/CD18 is part of the integrin family and is involved in inflammatory and immunological functions.
Yet another receptor molecule present on monocytes is CD163. It is identified as a member of the scavenger receptor cystein-rich superfamily (SRCR) present on cells of the monocytic family, such as most macrophages. Ritter et al., 1999 discuss the regulation, promoter structure and genomic organisation of the CD163 receptor. The precise function of CD163 is not disclosed. Furthermore, previous work on the biological function of CD163 is limited to a study on the effect of antibody-mediated crosslinking of CD163 on cultured monocytes (Van den Heuvel, M. M. et al. Regulation of CD163 on human macrophages: cross-linking of CD163 induces signalling and activation. J. Leukoc.Bil. 66, 858-866 (1999). The CD163 surface ligation induces a tyrosine kinase dependent signal resulting in intracellular calcium mobilisation, inositol triphosphate production, and increased secretion of anti-inflammatory cytokines.